UnitedHealth is testing the last major system it must restore from last month’s Change Healthcare cyberattack. The health care giant said Monday that it is testing software for submitting medical claims. It already has largely restored systems for handling pharmacy claims and processing payments. The American Hospital Association has called the cyberattack “the most significant and consequential incident of its kind against the U.S. health care system.” The association said the disruption from the attack made it harder for hospitals to provide care, submit insurance claims, and get paid.